Garbo-esque neutrinos may challenge
Standard Model

"I want to be left alone." ­ Greta
Garbo in the 1932 movie "Grand Hotel"

Shakespeare said that all the world's a stage
and all the men and women merely players. But to
Stanford physicist Stanley Wojcicki, all the
world's a laboratory, and its star player is the
neutrino, a subatomic particle that could rightly
be called the Greta Garbo of the physics world,
for it hardly ever interacts.

In 2003, Wojcicki and about 250 scientists
from 30 institutions representing five countries
will shoot neutrinos from an accelerator at
Fermilab in Batavia, Ill., to an underground
detector in a former mine 730 kilometers away in
Soudan, Minn. The path of the commuter neutrinos
is entirely underground, where they will interact
with nothing.

On Feb. 20 at the annual meeting of the
American Association for the

Advancement of Science in Washington, D.C.,
Wojcicki spoke about long-baseline

neutrino experiments, which are providing the
first crack in the Standard Model, the prevailing
theory that describes elementary particles and
forces. Right now physicists think everything is
made of 12 fundamental constituents ­ six called
quarks, six called leptons. Three of the leptons
are neutrinos. (Of the three particles taught in
high-school physics ­ electrons, protons, and
neutrons ­ only the electron is fundamental. It
is a lepton, whereas protons and neutrons are
each made of three quarks.)

The long-baseline experiments will explore
whether neutrinos have zero mass, as the Standard
Model says, and whether lepton number ­ a
property thought to be immutable like positive or
negative charge ­ is absolutely conserved.
Recent Japanese experiments using neutrinos
produced by high-energy cosmic rays suggest that
neutrinos do have mass.

"If these results are verified, neutrino
physics would be the first area to indicate
violation of the Standard Model because this
theory incorporates both lepton conservation and
zero neutrino mass," Wojcicki said."So
you have to modify the Standard Model to
accommodate these kinds of things." If the
experiments reveal, for instance, that neutrinos
have mass, however tiny, that discovery will
force physicists to paint a new picture of the
universe.

"It's very exciting from that point of
view that you really may be probing very, very
new physics," Wojcicki said. "You
really may have your hands on a smoking gun, and
if you pursue it, you can really learn more about
the murder, if you like."

Wolfgang Pauli first proposed the existence of
the neutrino in 1930 to explain an apparent
nonconservation of energy when radioactive
particles decayed. "I have committed the
cardinal sin of a theorist," he is reputed
to have said. "I made a prediction which can
never be tested, ever, because this particle is
so weakly interacting that it may never be
seen."

But 26 years later American scientists
Frederick Reines and Clyde Cowan detected the
first neutrinos. The next year, Italian physicist
Bruno Pontecorvo theorized that if different
species of neutrinos exist, they might be able to
oscillate back and forth between the different
species. In 1962, scientists from Columbia
University and Brookhaven National Laboratory
demonstrated the existence of two species of
neutrinos, and a third was found at Stanford
Linear Accelerator Center (SLAC) in 1975.

Experiments at the European physics laboratory
CERN and at Stanford in 1989 showed that only
three species of light (or massless) neutrinos
can exist: electron, muon and tau.

Neutrino oscillations transmute one kind of
neutrino, such as muon, into another kind, such
as electron or tau. But for oscillations to
occur, two phenomena must be true: neutrinos must
have mass, and lepton number cannot be absolutely
conserved.

To solve the mystery of whether neutrinos have
mass, physicists performed sophisticated
measurements, but all they got was an upper
limit. "So we know that neutrinos have a
very low mass, if they do have a mass," said
Wojcicki.

Regarding whether lepton number (charge) is
conserved, neutrinos have no charge that
physicists can measure. Here's where the story
resembles a detective finding a smoking gun at a
crime scene and using deductive logic to
reconstruct the murder. When electron neutrinos
interact, they make electrons, which do have a
charge. Similarly, muon neutrinos interact to
make muons, and tau neutrinos interact to make
taus. "Knowing what is the lepton in the
final state tells you about what kind of neutrino
was there initially," explained Wojcicki.

This is all well and good but does not explain
why physicists need to shoot neutrinos through
the curvature of the Earth from one state to
another, as in the Illinois-Minnesota experiment,
or from one country to another, as in a similar
Switzerland-Italy experiment that CERN will
conduct in 2005. (Japan is currently conducting a
modest accelerator experiment with a more limited
goal of determining whether scientists can
observe any oscillations at all.)

"We really want to do very precise,
quantitative studies to measure the mass
difference, measure the mode of oscillations,
what is the final state, and all of this,"
Wojcicki said.

To do that, scientists look at the probability
that one type of neutrino will change into
another type. The probability of that
transmutation oscillates like a wave, and it
increases as the distance grows between where the
particle is created and where it is detected. It
also depends on the difference in mass between
the first type of neutrino and the second. Since
the probability can be anywhere from zero to one,
if the difference in mass is small (as it would
be if particles have little or no mass), the
distance between particle birthplace and detector
­ called the baseline ­ must be large.

Though Wojcicki's research base is SLAC in
California, he travels to Illinois's Fermilab for
his neutrinos. His project, called MINOS, for
Main Injector Neutrino

Oscillation Search, requires more neutrinos
than SLAC can produce. "You need to have a
very intense neutrino beam because first of all
neutrinos do not interact very much, and secondly
you'll be detecting them very, very far
away," Wojcicki said. "It's like a beam
of flashlight where the farther you go, the
fainter is the light."

Accelerators allow scientists to pulse the
beam. "It's like a pulsar that emits bursts
of energy on a periodic basis," Wojcicki
explained. So while the background noise from
natural radiation sources, such as cosmic rays,
will be constant, the neutrino signal will be
pulsed from the direction of Fermilab. The
underground location of the detector shields it
from cosmic rays, reducing the background
significantly.

"The way it works is the accelerator
accelerates protons to higher energy,"
Wojcicki said. "Then protons are allowed to
strike the target. They make short-lived
particles called pions. You make a beam of the
pions, and they travel through a vacuum pipe
about half a mile long. As they travel, some of
them will decay, and they will decay into
neutrinos."

Another class of long-baseline experiments
employs nuclear reactors to study electron
neutrinos. (Whereas accelerators produce muon
neutrinos with ease, reactors are a good source
of electron neutrinos.) With reactor-generated
neutrinos, the detector need only be as far as
one kilometer away, as these neutrinos have a
much lower energy than their accelerator
siblings. (Energy is inversely proportional to
the odds that one kind of neutrino will transmute
into another kind, so the lower the energy, the
shorter the baseline.) Stanford Associate
Professor Giorgio Gratta is working on an
experiment in Japan that will look at neutrinos
from all reactors in Japan that, serendipitously,
happen to be far enough away from a central
detector to be useful (most are more than 100
kilometers away).

The cost of the MINOS program is about $120 to
$130 million, according to Wojcicki, with about
$70 million to build the Illinois infrastructure
and equipment and about $50 million for the
Minnesota detector. But unlike a single-purpose
experiment, such as a clinical trial designed to
find out if a specific drug is effective in
treating a disease, long-baseline experiments may
satisfy many goals. "The simplest
measurement, which is just to look at the muons
and compare the rates at the far detector and the
near detector, we can do in a few months or a
year," Wojcicki said. "More
sophisticated measurements ­ the detailed,
quantitative measurements, or looking for the
different modes ­ may take three, five
years."

Although Wojcicki called his research
"very, very basic," it has spawned a
practical application: a new technique for making
inexpensive scintillator, or material that
produces light when charged particles pass
through it and that is used extensively in
medical research today. His experiment will use
an unprecedented 250 tons.

New knowledge of neutrinos also may benefit
scientists in other fields. For example, it could
help cosmologists understand the evolution of
supernovae.

In May, neutrino physicists from around the
world will meet in Monterey, Calif., to discuss
experiments with even longer baselines ­ ones
that join one continent to another. Such
experiments face considerable technical
challenges, such as building a very intense
proton source, creating targets that can tolerate
intense beams, and concentrating beam particles
so that they can be guided around a storage ring.

Wojcicki is official spokesperson for the
MINOS project. Of the 250 people involved
worldwide, most have doctoral or advanced
engineering degrees. While only 15 are graduate
students, Wojcicki expects that number to triple
in the next two years because the scheduled date
to begin data collection makes the involvement of
graduate students, who usually stay four to six
years, appropriate at that time. Stanford
researchers are largely involved in the project's
software issues and include senior research
associate George Irwin and postdoctoral
researchers Robert Hatcher, Larry Wai and Carlos
Arroyo. Wojcicki also has worked with several
undergraduate students, including Brad Patterson,
who is working on an honors thesis. "There's
a strong educational component in addition to the
research objective," Wojcicki said.